This invention relates to control circuits for automatically refilling a container of dry granular or powdered material, e.g., foundry sand, and for maintaining the level of the material between a high level probe and a low level probe which are mounted at corresponding locations within the container. Control apparatus of this general type have been manufactured in the past, but the prior control devices have the disadvantage of requiring separate amplifiers for the high level probe and low level probe. Also, the prior control devices are limited with respect to the length of cable that can be used for coupling the probes to their respective amplifiers. This is due to the fact that beyond a limiting length, the capacitance of long cables exceeds the range of input capacitance for which these prior circuits will operate accurately.